1. Field of the Invention
The invention relates to flame-retardant thermoplastic materials which are prepared from polymers or copolymers of styrene and which contain an organic halogen compound. More particularly, these polymers incorporate a bromine compound as the flame retardant and a halogen-free synergistic compound having labile --C--C-- linkages. These synergistic compounds contain two nitrogen atoms, each of which is part of a heterocyclic ring system and both of which are bonded to the labile --C--C--bridge member.
2. Description of the Prior Art
The most pertinent prior art, which teaches the use of flame retardants and synergists for the flame retardants in thermoplastic molded materials, are the following: (1) German Application 1,244,395, (2) German Application 2,525,697, and (3) German Published Application 2,737,913.
The state-of-the-art is covered extensively and is summarized in (1) (see column 1, paragraphs 1-4). In order to improve the effect of the bromine-containing flame retardants, it is taught in (1) that synergistic compounds in amounts of 0.01 to 5 percent by weight based on the molded material, are added to the flame-retardant compound. These compounds containing labile linkages, which have a synergistic effect with the flame retardant, may contain chlorine, bromine, cyanogens, or nitro groups as substituents. It is taught that the synergistic compounds increase the effect of the bromine-containing flame retardants. The use of dehydropolymers of mono-, di- or trisubstituted benzene, which contain a cycloaliphatic hydrocarbon radical with 5 or 6 carbon atoms as synergistic compounds when employed in amounts of 0.05 to 2 percent by weight, is known from (2).
In (3) the use of halogen-containing flame retardants which are based on N,N'-alkylene-bis-tetrabromphthalimides is taught, wherein the alkylene group contains 2 to 15 carbon atoms. This compound class is a flame retardant containing a minimum content of about 35 to 40 percent bromine and which must be used in considerable quantities, namely, in quantities of 1 to 25 percent by weight based on the weight of the material containing the flame-retarding agent. The synergistic compounds recommended for these flame retardants are zinc borate, sodium tetraborate, lead arsenate, and similar materials, particularly, antimony oxide. The compounds taught in (3) are thermally-stable compounds which do not decompose at high temperatures or under the influence of light.
The synergistic compounds known from (1) have the drawback that they contain additional halogen, cyanogen, and/or nitro groups in addition to the bromine contained in the flame retardant. In the case of a fire, these compounds can cause considerable secondary damage due to the corrosive gasses evolved. Recently, there have been attempts to reduce the quantity of substituents forming corrosive gasses in the combination of flame retardant and synergistic compound. The teachings of reference (2) also point in this direction.
The purpose of this invention, therefore, was to reduce the bromine content of the flame retardant plus the synergistic compound as compared to the systems taught by (1) while retaining those good flame extinguishing times as taught by (1). According to the instant invention, shorter flame extinguishing times are achieved, when compared with the systems taught by (2). In comparison with the synergistic compounds taught by (2), it is also desirable to make available additional halogen-free synergistic compounds which can be produced cheaply and easily.